1896

Structure of the Atmosphere

Léon Teisserenc de Bort (1855–1913), Richard Aßmann (1845–1918)

The advent of balloon-borne remote sensing in the mid-nineteenth century and improvements in meteorological instruments and the balloons themselves enabled researchers to make direct measurements of the temperature and pressure of the Earth’s atmosphere up to very high altitudes. The pioneers in this new field of aerology (study of the structure of the atmosphere) were the French meteorologist Léon Teisserenc de Bort and the German meteorologist Richard Aßmann, contemporaries who were independently using high-altitude hydrogen balloons to measure the structure of the Earth’s atmosphere. Starting in 1896, for example, de Bort began launching hundreds of instrumented, unmanned weather balloons high into the atmosphere. Balloons at the time could operate up to altitudes of about 56,000 feet (17,000 meters).

These early pioneers discovered that the atmosphere is divided into at least two distinct layers. In the lowest layer close to the surface, which de Bort called the troposphere (from the Greek “turning”), the temperature slowly decreases with altitude up to around 40,000 feet (12,000 meters). Above that in the higher layer, however, the temperatures remained relatively constant for as high as their balloons could fly. De Bort called this upper zone the stratosphere (from the Greek “spreading out”), and the boundary between the layers the tropopause.

Subsequent weather balloons, high-altitude aircraft, suborbital sounding rockets, and eventually orbital satellites have significantly enhanced our understanding of the structure of our planet’s atmosphere. Rather than just two main layers, Earth’s atmosphere actually has five. From bottom to top, these are: (1) the troposphere, going from the surface to about 7 miles (12 kilometers) altitude and containing about 80 percent of the mass of the atmosphere; (2) the stratosphere, which was eventually discovered to extend up to 31 miles (50 kilometers) and to get significantly warmer above the limit of early balloons because of heating by atmospheric ozone; (3) the mesosphere, extending up to 50 miles (80 kilometers), and where temperatures once again decrease with altitude; (4) the thermosphere, extending up to 440 miles (700 kilometers), and where the solar wind and Earth’s magnetic fields can ionize this rarefied air and produce auroral displays; and finally (5) the exosphere, extending up to about 6,200 miles (10,000 kilometers), and where atoms and molecules easily escape and return to our atmosphere along those same magnetic field lines.

Photo of the hand-guided release of a typical US Weather Service atmospheric sounding balloon, sometime between 1909 and 1920.